In paper machines and similar devices, in which a continuous material web is manufactured or employed, it is often necessary to identify the presence of the material web or the location of its edge in various stages of the process. For these purposes, as a rule, photoelectric means of identification are used. The prior art devices for identification of the web usually operate so that the source of light and the photocell are placed on opposite sides of the web to be monitored, and a break of the web and/or shifting of the edge results in the photocell receiving the beam of light, and as a result transmits an impulse, which results in the alarm and possibly in other action.
In addition, various devices of identification are known which are based on reflection of light from the material to be monitored and on changes occurring in the nature of the reflection. As an example of such devices, reference is made to the U.S. Pat. No. 4,146,797, wherein a device for identification of the location of the edge of a material web is described. This device comprises a source of light and a detector of light. The source of light acts to direct a spot of light on the lateral area of the web to be monitored. The position of the lateral area of the web is monitored and detected on the basis of changes taking place in the intensity of the reflected light.
The prior-art photoelectric devices of identification do no operate adequately under all conditions, and are subject to disturbances. They require constant supervision, frequent calibration and cleaning. In particular, the conditions present in paper machines are an operational environment that imposes very strenuous requirements on transducers because of high temperature, moisture and impurities, which produce errors and false readings in the prior-art photoelectric means of identification. With increasing speeds of paper machines, these problems have increased further.
For example, in a paper machine, a false-positive break alarm with the resulting restarting operations usually causes a standstill of at least about one hour, because the restarting of the paper machine requires a number of steps, including the threading of the web through the machine. Thus, with the prior-art devices, false-positive or false-negative alarms cause considerable economic losses and a reduction of the overall efficiency of operation of the paper machine. A particular problem occurs in areas of single-wire draw in the drying sections of paper machines, where the web is constantly supported by a drying wire. In such a case, by means of the identification device, it is necessary to be able to distinguish the web from the drying area, and a source of light and a photocell placed on opposite sides of the web and the wire cannot be used, and it is necessary to resort to the reflected light from the object to be examined. Moreover, contamination of the wire and variations in the color and the moisture content of the paper web to be detected cause changes in the intensity of the reflected beam of light, making the identification by means of the prior-art devices uncertain.
The operation of the prior-ann web-identification devices is also disturbed by background light, which may vary in intensity, and may include oscillations of fluorescent lamps arising from the line frequency. In the environment of a paper machine, there is also a considerable amounts of infrared radiation, to which most photocells are sensitive, which also causes disturbances in the photoelectric identification device.